PhD Oral Exam - Kaue Bertuol, Mechanical Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

The increasing demand for fuel efficiency and reduced environmental impact in aircraft propulsion has accelerated the development of advanced clearance control systems for gas turbine engines. Abradable seal coatings, applied to minimize the gap between rotating blade tips and stationary casings, reduce compressed air leakage, enhance operational safety, and lower fuel consumption, CO₂, and NOₓ emissions. In low-pressure compressor (LPC) applications, thermally sprayed aluminum–silicon (AlSi) alloys, often combined with solid lubricants and engineered porosity, offer a balance between abradability and durability under harsh operating conditions. However, due to the need for sophisticated custom-made abradable test rigs, developing and evaluating novel abradable materials under application-relevant conditions is challenging, typically very costly and time-consuming. Thus, this research addresses that gap by delivering a multi-scale evaluation of AlSi-based abradable coatings, linking fundamental wear mechanisms with application-relevant performance. A cost-effective abradable test rig was designed and validated to replicate LPC contact conditions, enabling rapid pre-screening of candidate materials. Three AlSi-based materials (AlSi-Poly, AlSi-hBN-Poly, and AlSi-MoCr-Poly) combined with different chemical compositions (hexagonal boron nitride, molybdenum and chromium) were thermally sprayed via Atmospheric Plasma Spraying (APS) and characterized through microstructural and tribological approaches. Wear performance was evaluated across multiple Technology Readiness Levels (TRLs), starting with ball-on-disk and ball-on-flat tests (TRL 3), and afterward compared with abradability tests conducted under harsh conditions using the custom-built abradable test rig designed in this study (TRL 4). Both room-temperature and elevated-temperature (300 °C) assessments revealed distinct coating filler effects and negligible blade wear. hBN addition combined with polyester reduction promoted higher friction coefficient, rubbing forces and frictional heating, but also demonstrated improved thermal stability and reduced sensitivity to temperature-induced frictional changes. MoCr additions produced negligible changes in wear behavior compared to the baseline, suggesting viability as corrosion-resistant modifiers. Temperature elevation significantly reduced reaction forces for all coatings due to polymeric phase softening, without inducing blade damage. Coating wear was dominated by abrasive cutting, with minimal aluminum adhesion to the titanium blades. Overall, this research provides new insights and practical tools for the development and evaluation of next-generation abradable materials, delivering benefits to both academia and industries engaged in advanced sealing technologies.